Synthetic resin bottles, and particularly PET bottles, have recently been produced in large volume for use as containers for beverages, foods and the like due to their light weight and handling ease.
PET bottles are obtained by so-called biaxial stretch blow molding consisting of first producing a primary molded article as a preform by injection molding, next placing the preform in a predetermined metal mold, heating the preform with a heater or other heating means to a temperature that allows the preform to be oriented and stretched, inserting a stretching rod thereinto that injects high-pressure air, and stretching the preform biaxially with the stretching rod and the high-pressure air. Since the biaxial stretch blow molding stretches the heated preform in an axial direction and a direction perpendicular thereto with the stretching rod, PET bottles may be produced having defects such as pinholes or cracks due to slight differences in processing force (stretching force) and contamination of extremely small particles in the resin material. Thus, PET bottles after blow molding requires an inspection to determine whether or not there are pinholes or other defects therein. A known example of an inspection method for this purpose consists of filling air in a PET bottle clamped along the outer periphery of a main rotor, sealing the air inside the bottle by contacting a pressure head with the mouth of the bottle, maintaining this airtight status for a fixed amount of time, and then assessing the bottle for the presence of pinholes based on a inner pressure reduction quantity of the bottle in elapse of the fixed amount of time (see, for example, Patent Document 1 and Patent Document 2).
FIG. 13 is a graph indicating a conventional pinhole inspection method in which the presence of pinholes is assessed based on a pressure reduction quantity. Bottle inner pressure [kPa] is plotted on the vertical axis, while valve sequence/time [sec] is plotted on the horizontal axis.
In this pinhole inspection method, air is supplied inside a bottle by opening a supply value at a time T=t0, for example. Next, the supply of air is stopped by closing the supply valve at a time T=t1, and a predetermined amount of air is sealed in the bottle. The airtight status is then held until a time T=t2. In presence of such a defect as a pinhole that breaks the air tight of the bottle, air flows to the outside of the bottle through the defect, which correspondingly results in a reduction of bottle inner pressure. However, as the temperature inside the bottle gradually lowers, the bottle inner pressure decreases in proportion thereto. In addition, bottle inner pressure also decreases due to slight swelling of the body of the bottle to the outside due to a rise in inner pressure. In other words, bottle inner pressure decreases to a certain degree in elapsed time even if there are no pinholes and so forth present in the bottle. Therefore, the pressure reduction quantity caused by a temperature decrease or other cause not attributable to pinholes is preliminarily defined in the form of a threshold value ΔP0, and the amount of a pressure reduction quantity from the time T=t1 to time T=t2 is inspected on the basis of the threshold value ΔP0. If the pressure reduction quantity does not exceed the threshold value ΔP0, the bottle is judged to be acceptable. On the other hand, in the case the pressure reduction quantity exceeds the threshold value ΔP0, the bottle is judged to be unacceptable. For example, since a pressure reduction quantity ΔP1 of a bottle 1 does not exceed the threshold value ΔP0, the bottle 1 is judged to be free of defects such as pinholes and be acceptable. On the other hand, since a pressure reduction quantity ΔP2 of a bottle 2 exceeds the threshold value ΔP0, the bottle 2 is judged to contain defects such as pinholes and be unacceptable. Air in the bottle is then discharged at the time T=t2, and the pinhole inspection ends after the passage of a predetermined amount of time at a time T=t3. PET bottles that have been judged to be acceptable are transferred to the next step of a sterilization and filling step by a transfer device after the bodies thereof have returned to their original shape. On the other hand, PET bottles that have been judged to be unacceptable are removed and not transferred to the next step.
Bottle inner pressure immediately after air sealing at the time T=t1 of a bottle 3 that contains a pinhole is obviously lower than inner pressure of another acceptable bottle (bottle 1) that is free of pinholes. This tendency becomes increasingly prominent in lightweight, thin-walled bottles in which the wall thickness of the body is thinner than that of conventional bottles.
However, regarding a pressure reduction quantity ΔP3 of the bottle 3, because of a low inner pressure immediately after air sealing, small quantity of air flows out through a pinhole. As a result, there can be cases in which the pressure reduction quantity ΔP3 from the time T=t1 to the time T=t2 does not exceed the threshold value ΔP0. In this case, there is the risk of the bottle 3 being judged to be acceptable according to this conventional inspection method despite the presence of a pinhole therein.
In addition, conventional pinhole inspection apparatuses do not have a function (self-diagnostic function) for confirming the inspection accuracy (pinhole detection accuracy) of the inspection apparatus itself, and the confirmation of the inspection accuracy of the inspection apparatus itself is left to an arbitrary judgment of a user.
In the conventional pinhole apparatus, user judges the inspection apparatus to maintain a required pinhole detection accuracy by periodically placing test pieces containing pseudo pinholes in the inspection apparatus and confirming that these test pieces are reliably rejected by the inspection apparatus. However, not only does this type of confirmation of inspection accuracy require considerable amounts of, time and labor for the task of confirming accuracy, but since the diameter of the pseudo pinholes gradually expand according to the using times of the test pieces, it is necessary to prepare a new test piece with accurate pseudo pinholes for each inspection in order to precisely confirm pinhole detection accuracy, whereby making this method far from rational. In addition, the placement of test pieces was also difficult in the case of production lines in which PET bottles are continuously transferred at high speeds.
In addition, these pinhole inspection apparatuses are provided separately and independently from blow molding machines. Thus, bottles immediately after blow molding are transferred from the exit of the blow molding machine to the entrance of the pinhole inspection apparatus by a conveyor or other type of transfer line. At this time, it is necessary to synchronize the timing at which bottles are fed to the pinhole inspection apparatus and the timing at which the pinhole inspection apparatus takes in the bottles. In the case these times are not in synchronization, the pinhole inspection apparatus ends up operating without containing any bottles, or bottles end up gathering and becoming jumbled at the entrance of the pinhole inspection apparatus, whereby leading to a undesirable situation in terms of production efficiency. In addition, such mechanisms as timing screws and gravity wheels are known to be used as timing adjustment devices for adjusting the timing (see, for example, Patent Document 3).
The timing at which a blow molding machine discharges bottles and the timing at which bottles are taken into the pinhole inspection apparatus are typically not synchronized. Thus, it is necessary to control the feed timing of bottles with a timing screw or other type of timing adjustment device so that the timing at which bottles are fed to the pinhole inspection apparatus is synchronized with the timing at which bottles are taken into the pinhole inspection apparatus.
However, in the case of a timing screw, there is the risk of bottles becoming caught in the threads thereof. In addition, in the case of transferring bottles from a blow molding machine to a pinhole inspection apparatus, there is also the risk of the bottles tumbling and shutting down the line. In conventional blow molding machines and pinhole inspection apparatuses, these problems are unrelated to the shape (cross-sectional shape) or strength (wall thickness), and can occur in bottles of any shape or strength.    Patent Document 1: Japanese Patent Application Laid-open No. 2004-205453    Patent Document 2: Japanese Patent Application Laid-open No. 2002-310843    Patent Document 3: Japanese Patent Application Laid-open No. 2004-26435